Carbon-carbon composites are a unique class of materials whose elevated properties make them attractive for various aerospace applications. Anticipated applications of carbon-carbon composites in gas turbine engines include components such as exhaust nozzle flaps and seals, and tail cones. The materials are composites, although in general, all of the composite elements are carbon, in its various allotropic forms. Carbon-carbon materials are produced from organic precursor fibers, such as polyacrylonitrile, rayon, or pitch. Such fibers are usually produced in bundles (yarn), often by an extrusion process. The precursor fibers are heated in an inert atmosphere to pyrolyze or carbonize them, and may then be heated to a higher temperature (usually above about 2,000.degree. C.) to form graphite fibers. These graphite fibers may then be layed down, woven, or interleaved to form what are referred to as 1D, 2D, 3D, etc., structures where D stands for direction (i.e., in a 2D structure, fibers are layed in two, usually orthogonal, directions).
These woven structures may then be impregnated with a pitch or resin material which, by the application of heat, is converted to carbon which acts as the matrix for the composite. Hot pressing may be employed to obtain a more dense structure. Repeated impregnation steps may also be used to increase the density of the product being fabricated. In general, the product never achieves 100% theoretical density, i.e., it is porous. The density of some carbon-carbon components is in the range of about 1.3-1.75 grams per cubic centimeter (g/cc).
The finished product is usually at least 90-95% carbon, but by virtue of the fiber alignment and other processing details, has exceptional mechanical properties when compared with other carbon type materials such as monolithic graphite. The mechanical properties of carbon-carbon composite materials are constant, or even increase slightly, with temperature exposure up to about 2,000.degree. C. This temperature capability makes carbon-carbon materials exceptionally attractive for various aerospace applications.
One of the few drawbacks of carbon-carbon materials is their susceptibility to oxidation degradation at temperatures above about 340.degree. C. Various coatings have been developed which protect carbon-carbon substrates from oxidation. To be fully protective of the carbon-carbon substrate, the coating must fully encapsulate the component, viz., there must be no gaps in the coating, or defects which extend through the thickness of the coating and to the substrate. In applications where the coated component will be exposed to severe cyclic conditions (temperature and/or stress), a duplex, or two layer, coating system may be used to insure that the substrate is sufficiently protected from the environment. Some of the useful coatings for carbon-carbon components are disclosed in, e.g., U.S. Pat. Nos. 4,465,777, 4,472,476, 4,476,164, 4,476,178, and 4,544,412, all of which are incorporated by reference. Glass-type coating systems for carbon-carbon components have also been proposed.
Cracks in single layer coating systems, as well as in two layer coating systems have been observed. While a majority of these cracks do not extend to the carbon-carbon substrate, some do. Catastrophic oxidation of the carbon-carbon substrate may occur when oxygen, moving through these defects, comes in contact with the substrate at elevated temperatures. Theoretically, any defect which is larger than the size of the oxygen atom is large enough to permit contact of oxygen with the substrate, which would begin the oxidation process. However, the rate at which oxygen moves through these very small through thickness defects is slow, and it is probably not until the component experiences stress at high temperatures that these defects increase to a size at which rapid oxygen diffusion, and therefore, rapid oxidation degradation, occurs. It is believed that the through thickness defect size which can result in rapid oxidation may be as small as about 15-150 microns. For the purposes of this specification and attached claims, defects which extend from the outer surface of the coating and through the entire thickness of the coating to the carbon-carbon substrate are termed through thickness defects.
Metallographic and X-ray techniques have heretofore been used to detect the existence of defects in coatings applied to carbon-carbon materials. However, as is known to those skilled in the art, such techniques are time consuming. As a result, newer and faster techniques are constantly being sought to determine the existence of such defects.
Numerous prior art patents teach methods for the nondestructive detection of cracks and other defects in various types of materials. See, e.g., U.S. Pat. Nos. 1,370,347, 1,613,962, 2,055,568, 2,316,842, 2,407,945, 3,064,466, 3,590,256, and 3,592,047. However, each of these patents is concerned with the examination of pressurized or air-tight containers, or, alternatively, the examination of articles using pressurized liquid. In both of these types of techniques, cracks are identified by air bubbles which rise from the surface of the article being examined. However, these techniques are complicated, and often use bulky equipment; they are not felt suitable for the examination of carbon-carbon composite materials, especially aircraft engine components, some of which can be several feet in length and breadth.
Thus, what is needed in the art is a fast, yet simple method for detecting very small, surface connected defects in coatings applied to carbon-carbon materials.